Processes and apparatus for preparing meltblown nonwoven webs from thermoplastic fibers have been described in publications such as Naval Research Laboratory Report No. 11437, submitted, Apr. 15, 1954; Naval Research Laboratory Report No. 5265, dated Feb. 11, 1979, and in a multitude of patents including U.S. Pat. Nos. 3,775,527, 3,825,379, 3,878,014, 3,849,241, and 3,978,185, to name but a few. In the melt blowing process, a molten polymer is extruded through a row of small orifices forming a row of fibers which are attenuated by convergent hot air streams on either side of the fiber row. The fibers are collected on a moving collector screen forming a web thereon. The equipment for carrying out the melt blowing process comprises an extruder, a die, air flow means, a collector screen, a windup drum, as well as ancillary components such as blowers, air heaters, and controls. The collector screen, as disclosed in U.S. Pat. Nos. 3,825,379 and 3,878,014, is frequently in the form of a rotating screen which collects the web on the surface and permits passage of air into the interior thereof.
The fibers must be evenly deposited on the collector screen to produce a uniform, high quality nonwoven web. One prior art approach for improving the web uniformity involves applying vacuum to the interior of the screen drum to increase the flow of air through the screen. Although the vacuum collector systems have been adequate for some applications, there remains a need for further improvement to produce high quality uniform webs, particularly for webs used as filters.